In a recent GRM test (August issue?) they used their MX-5 to do a wheel weight and tire size test on lap times at The Tire Rack. They found that, at least in an Autocross type setting that 4 lb. lighter wheels made almost no difference.

They then, using Star Specs, tested 255/40R17 vs. 245/40R17 or what they described as 'wider and taller' vs. 'shorter and narrower' tires. They found the 245 setup to be much faster with drivers reporting the feel being more precise and nimble.

I'm confused by this as I generally thought we want to stuff as much rubber as possible under each corner. Is this just an autocross thing ( I just do HDPE)?

 

Also note that the GRM drivers were slower at Nats than a bunch of our fast STR S2000 drivers, so take their findings with grains of salt. If it made a tremendous difference, you'd think they would have done better than the S2000s (who pretty much exclusively ran 255s).

 

Hard to say for sure that the top S2000 drivers couldn't have been even faster on 245 tires.

What's really too bad is that there isn't a 255/35R17 StarSpec or RS3 that would certainly be a faster tire.

 

That size would be faster for AP1's but I think it would reduce the top speed in 2nd too much on AP2's...

I have tested (STR S2000) with both 245 and 255 RS3's on 17x9 volks (although unfortunately not back to back) THe 245's are a little easier to drive, and more responsive, they also heat up quicker which is good on cooler days. BUT once at temp the 255's have more lateral grip. IMHO the difference in performance was very small, but with the exception of cooler days on very transition heavy courses, the 255's are faster. FOR HPDE i imagine the 255 will always be faster since you get up to temp every time.

 

A bunch of our local STX Bimmer guys extensively tested 245 vs 255 Star Specs and 245 vs 255 RS3s. They were consistently faster on the 255s if they had a co-driver and could get heat in them. The main tester is a Nationals 3x runner up and a hardcore enginerd who sets up a bajillion sensors in his cars to his DL1.

 

Thanks all,

This really helps me square this in my mind. I'm not an Autox guy, and this seems to be VERY oriented to the short run / quick transition / fling-ability needs of Autox vs. longer runs and max. lateral G's one looks for on road courses.